The invention relates to an optoelectronic device comprising a semiconductor diode laser amplifier, often referred to as amplifier for short hereinafter, with a strip-shaped active region which is bounded in longitudinal direction by two end faces where radiation can enter or leave the amplifier, while at one side of the amplifier are present in that order: a lens, a polarization-changing element, and a mirror, such that radiation passing through said element, reflected by the mirror, and passing through the element once more has a polarization which has been rotated through 90.degree.. In general, an amplifier will have a gain factor which is greater than or equal to one. It is noted, however, that a gain factor smaller than or equal to one is also possible, indeed useful under certain circumstances, during operation. It is also possible for the gain factor to be varied in time, such as is the case, for example, when the amplifier is used as a modulator.
Such a device is used, for example, in an optical glass fiber communication system. The device is particularly suitable for use in the wavelength bands around 1.3 .mu.m and 1.5 .mu.m.
Such a device is known from the article "Polarization-independent configuration optical amplifier" by N. A. Olsson, published in Electronics Letters, 18th August 1988, vol. 24, No. 17, pp. 1075-1076. The device shown therein (see FIG. 1) comprises a semiconductor diode laser amplifier with two end faces where radiation can enter or leave the amplifier. The radiation to be amplified enters the amplifier at one side thereof, and the radiation leaves the amplifier at the other side after being amplified a first time, subsequently passes through a collimating lens and a Faraday rotator which rotates the polarization through 45.degree.. The radiation is subsequently reflected by a mirror, whereupon it follows the above path in reverse order, during which the polarization is once more rotated through 45.degree. upon passing the Faraday rotator. The radiation finally leaves the amplifier, after a second amplification, at the same side where the radiation to be amplified entered the, amplifier. The radiation to be amplified accordingly passes twice through the amplifier, while the polarization of the radiation passing through the amplifier for the second time has been rotated through 90.degree. compared with the polarization of the first passage. A polarization sensitivity, which is always present in the amplifier and which means that the gain is dependent on the polarization, is neutralized thereby. This means that the final gain achieved no longer depends on the polarization of the radiation to be amplified. In other words, the amplifier is polarization-insensitive in the given configuration. This is of major importance in a number of applications, such as wavelength multiplexing, or more in general applications where it is not possible to keep the gain constant by means of an (opto)electronic feedback control mechanism because of the necessary very high switching speeds.
A disadvantage of the known device is that it has a comparatively low efficiency in practice. This means that the amplifier must be operated at a comparatively high gain factor, which involves several further disadvantages. It is also found that the efficiency varies from one device to another.